A pneumatically driven ram boring machine of the above-mentioned kind is known from German patent specification No. 21 57 259. It has a moveable impact tip on which the self-guiding piston strikes periodical hammer-blows. Under the influence of the hammer-blows the impact tip, which is buffered on the machine housing via a compression spring, advances oscillatingly through the ground and finally, when it comes to the end of its stroke, it draws the housing after it.
The impact tip of the ram drilling machine can be formed as a chisel or, according to German Offenlegungsschrift No. 25 58 842, can be provided with blade-like cutters so as to break up any obstructions in the ground more easily. Furthermore such ram boring machines can also be combined at the rear end with a coupling for attaching a follow-up pipe. In this way it is possible to pull a supply pipe even through ground crossed by obstructions such as old pipes without trenching.
From German patent specification No. 28 24 915 it is also known to use such ram boring machines to widen out a pilot hole, which may even be an existing supply pipe. Since the widening out of the supply pipe, for example a cast iron or earthenware pipe, requires it first to be broken up, it is also known to provide the impact tip with cutter or impact blades pointing radially outwards or arranged in the axial plane of the impact tip. In this case the cutting edges run at an acute angle to the axis of the impact tip, so that they act together much like a spreader cone but break apart the wall of the previously-laid pipe as the impact tip moves forwards.
Since the impact tip and the cutters form a constructional unit that is either part of a ram boring machine or is connected to a ram boring machine, the operations of driving forwards, forcing aside and breaking up are always performed at the same time. This results either in increased impact energy or in a slower advance. Moreover the impact energy is frequently not enough to break apart the sleeve joints that are almost always present.